1. Field of the Invention
The invention relates to a process and a device for localizing targets and focusing acoustic waves (the word "acoustic" being understood in a broad sense, without limitation of the audible frequency range) on targets which have an acoustic impedance significantly different from that of the environment. Such targets include solid objects in a sea environment or in sediments at the bottom of water bodies, cavities close to the earth surface. However, the invention is particularly suitable for use in localizing and possibly destructing calculi in human tissues, particularly the kidneys, the vesicle, the bladder and the urethra, as well as detection and localization of flaws, faults and heretogeneities in various materials, such as metals, composite materials and ceramics.
2. Prior Art
Lithotripty devices are already known comprising an ultrasonic generator whose transmitter is a rectangular or circular transducer array energized so as to generate an ultrasonic field focused on the assumed position of the calculus.
Due to the acoustic heterogeneity of the human tissues, knowledge of the geometrical position of a calculus, obtained for example by X-rays, is not sufficient to determine which time delays between pulses energizing the transducers or distribution in space of these transducers, will result in accurate focusing on the calculus. Beams must consequently be used whose focus spot is relatively wide, often about 1 cm.sup.2. To obtain an energy density sufficient to destroy a calculus, powerful sources must be used which are cumbersome and expensive. And the lack of centering accuracy may damage tissues close to the calculus due to heating, all the more so since the calculus may have moved between the time of localizing it and the time of applying the destruction beam.
It might be thought that the problem can be overcome by using a lithotripty device which further comprises means for forming an echographic image by means of a transducer array (U.S. Pat. No. 4,526,168). But, in practice, the additional means do not solve the problem since applying energization pulses to the different transducers with the same distribution of delays as that which corresponds to focusing on a detected calculus does not concentrate the whole energy on the calulus, which has finite dimensions and a shape which may be irregular.
It is an object of the invention is to provide a process and device for accurately focusing an ultrasonic beam on a target of high reflectivity, such as a calculus, and to provide self-adaptation of the wave front to the shape and position of the target itself, possibly for the purpose of destroying it, whatever the shape of an interface or of interfaces between the array of transducers providing the ultrasonic beam and the target.
For that, the invention uses a technique which may be termed "phase conjugation sound amplification" due to some analogy with phase conjugation mirrors used in optics. With this technique, starting from an incident ultrasonic wave which diverges from a target (a calculus reflecting a beam which it receives, for example) a convergent wave may be formed which follows exactly the opposite path, possible distortions being compensated for. For that, the divergent wave is received on an array of detectors, the signals received are reversed in time as regards both their shape and their time distribution (i.e. their laws of variation in time are reversed and their orders of occurence are reversed) and the signals thus reversed are applied to the array.
In most cases, the target will constitute a secondary source, which reflects or scatters a wave beam applied to it. The target may for instance consist of:
a stone reflecting a beam received from an array of illumination transducers, a small size tumor impregnated with a contrast agent (which renders it possible to carry out ultrasonic hyperthermia), a fault in a solid object,
a small size cavity within the ground, close to the surface.
An initial illumination of the zone where the target is expected to be found will provide an indication on the boundaries of the latter, which has an acoustic impedance very different from that of the surrounding and which will appear as a secondary source.
Consequently, there is provided a process according to the invention, including the steps of:
(a) illuminating a zone including a target to be detected with an unfocused beam;
(b) individually storing the shapes and positions of echo signals delivered by the transducers of an array; and
(c) reversing the distribution in time and the shapes of the signals for obtaining reversed signals and applying said reversed signal to the respective transducers.
Echoes which are originally quite low may be progressively amplified, by applying an amplification coefficient at each reversal.
Possibly some or all transducers of the array are used, rather than separate means, for illuminating the zone during step (a).
The process may use iteration, by repeating a sequence consisting of steps (b) and (c), after a first illumination of the zone concerned in which a target is sought, which has a reflectivity higher than the average reflectivity of the environment. Each time reversal of the echo enhances the ratio between the energy reflected by the target of high reflectivity and the energy reflected or scattered by local irregularities.
The final step of the process may consist of recording and/or displaying the final sound wave front. However, when the process of the invention is used for destructing stones in tissues, such destruction may be made by focusing ultrasound energy with the same array or with an other array which is better adapted to delivery of a high amount of energy. In both cases, the wave front of the destruction beam will reproduce the recorded and/or displayed wave front. Once the final wave front has been recorded, it will often be sufficient to use only the time distribution of the first maxima received in return by each of the transducers of the array after several iterations and to energize the destruction transducer array by simply respecting such time distribution, while disregarding the secondary lobes of the signal received and stored in digital form. Identification of the maximum of each signal and of its location in time raises no problem for well-known digital techniques exist for that purpose, using analog or digital correlators now commercially available. In some cases, it may however be advantageous to determine the time delay function to be respected, by evaluating the cross-correlations between a pair of signals.
Another solution consists in using, for destruction purpose, electronics which are distinct from the localization electronics but receive the information stored during localization, the same transducer array being used for localization and destruction.
During step (c) of the last sequence, an amplification gain is adopted such that destruction of the calculus is caused if such a purpose is to be obtained while the energy transmitted during localization may possibly be low.
It is important to note that the process which has just been described achieves a progressive selfadaptation of the wave form to the shape of the target; the distribution in time of the signals applied to the transducers and the shapes of the signals finally reflect exactly the shape of the calculus.
The process as hereinbefore described makes it possible to focus an ultrasound beam on the target which has a maximum reflectivity in an environment (or on a plurality of strongly reflecting targets if they are mutually spaced). Such focusing, when used for display purpose, "erases" the targets which have a lesser reflectivity and are masked by the target (or the targets) of maximum reflectivity. After a strongly reflecting target has been detected and localized, it may be useful to detect and localize targets which have a lesser reflectivity and which were masked by a main target during the initial focusing process. For that purpose, there is provided a process which includes, after a first focusing sequence as defined hereinabove, an additional sequence including the steps of:
(a1) illuminating the zone including the previously localized target with a non-focused wave beam;
(b1) collecting and storing echo signals received by the transducers of the array and individually storing the shapes and positions in time of said echo signals;
(b2) individually subtracting the echo signals received by each transducer of the array and stored during the last step (b) as defined above from the stored echo signals obtained during step (b1);
(c1) reversing the distribution time and the shapes of the results of the subtraction for obtaining further reversed signals and applying said further reversed signals to the respective transducers.
By repeating steps (b1) and (c1), the target whose reflectivity level is immediately lower than that of the target localized during the first sequence of operation may be detected and its position may be determined.
The above-defined process for detecting less reflective targets is simple but has a limitation: the final localization of the most reflecting target during the first sequence results from n transmission-reception sequences (n being an integer typically greater than 1) and the transfer function of the transducers modifies the echo signal upon each sequence. For removing the perturbating effect due to the accumulated distortions, step (b1) may include the additional phase consisting in subjecting the echo signals received by the transducers to n convolution operations, each corresponding to a transmission-reception sequence, before the echo signals representing the wave front on the most reflective object are subtracted.
Rather than carrying n convolution operations on the first reflected wave front, it is possible to carry out n deconvolution operations on the stored wave front.
The invention also provides a device for implementing the above-defined process, comprising: a transducer array; and, associated with each transducer of the array, a processing channel comprising an A/D converter, memory means, a programmable power transmitter controlled by the memory means, and means for energizing the transmitters in accordance with a time distribution which is reverse of the distribution stored in said memory means.
The device may be complemented with echography means for displaying the echo producing targets in the observation zone; targets on which the energy will subsequently be focused may be selected, for example by selecting some only of the transducers for later use.
Due to this arrangement, the energy applied to the transducers is used under much better conditions than in the past. In particular, if calculus is small of size, the focal spot may be reduced to the minimum allowed by diffraction and which, for an ultrasonic frequency of 1 MHz, is a few mm.sup.2. It is not necessary that the transducers be accurately positioned in the array, for the time reversal accomodates possible positioning errors.
The invention will be better understood from the following description of particular embodiments, given by way of non-limitative examples.